Skeletal muscle has the ability to change fiber size, contractile velocity, and myosin composition in response to a wide variety of stimuli. This renewal application will continue to focus on the properties and functions of the myosin heavy chain (MyHC) motor proteins that are expressed in skeletal muscle and that play a role in this plasticity. In mammals, striated muscle MyHCs are encoded by at least 9 closely related genes. Recently, 23 different mutations in the embryonic, perinatal, Ha, and (3 MyHC genes have been shown to cause 6 distinct skeletal myopathies. However, the mechanisms by which these mutations cause disease and their specific effects on each protein are not understood. We have recently achieved the long-sought goal of expressing and purifying recombinant, enzymatically active human skeletal myosin motor domains (both wild type and disease-causing mutants). Here, we propose a tripartite study of skeletal myosins and disease-causing mutations in them: In Aim I, these recombinant motors will be studied biochemically and biophysically. In Aim II, full-length myosins will be studied in a cultured cell system for the effects of mutations on sarcomere structure and stability. In Aim III we propose studies in functioning skeletal muscles. We have established transgenic nematodes for characterizing this relatively large number of disease-causing alleles rapidly;the most interesting subset of mutations will then be analyzed in transgenic mice. In previous work funded by this grant, we generated mice that were null for MyHCs lid and Mb;their properties have revealed a great deal about the regulation and function of these genes. Mice null for the MyHC Ma gene have recently been obtained by transposon mutagenesis and provided to us. Our early work shows that they are quite distinct from the other myosin nulls. Here we propose to analyze MyHC Ma null mice to complete the genetic analysis of all 3 adult fast MyHC genes. We will test the hypothesis that absence of MyHC lla results in respiratory dysfunction and an inability of muscle to adapt to exercise. Lay Statement: Muscle function is essential for activities such as breathing, walking, eating and for the beating of the heart. Many diseases affect muscle function and our work focuses on how normal muscle works and how mutant genes caused muscle disease.